1. Technical Field of the Invention
This invention is concerned with a means for applying an adjustable mechanical damping force to a moving-coil geophone.
2. Description of the Prior Art
Moving-coil geophones are commonly used in geophysical exploration to measure particle velocity of the earth due to seismic waves. A typical geophone consists essentially of an induction coil of fine insulated wire, resiliently suspended in an air gap between concentric pole pieces of a permanent magnet. Relative motion between the coil and the magnetic field induces a voltage in the coil proportional to the velocity of the relative motion. One such geophone is disclosed in U.S. Pat. No. 3,913,063 to Sears, this patent being incorporated herein by reference as an example of a commercial instrument.
A geophone is an oscillatory system. All such systems resonate at their natural frequency if undamped. If the damping factor is zero, the system will theoretically oscillate with infinite amplitude. The system will not oscillate if the damping factor is unity; it is then said to be critically damped. For geophysical operations, a damping factor of less than 1.0, or about 0.6 to 0.7 critical, is desired.
Absent external damping effects, certain geophone damping forces are inherent in the manufacture of the geophone. Damping may be mechanical, electrical, or viscous. The mechanical damping factor is inversely proportional to the combined mass of the coil and the bobbin upon which it is wound. If the bobbin includes a metallic portion, an induced EMF and Foucault or eddy currents are generated therein. The resultant current flow creates an electrical damping force that is directly proportional to the magnitude of the induced and eddy currents. The fluid in which the coil and bobbin are immersed, creates viscous damping. Since most commercial geophones are air-filled, rather than liquid-filled, viscous damping is negligible and can be ignored.
Mechanical damping and electrical damping are inversely related. Thus, if a larger mass is applied to the coil and bobbin to reduce the damping, the electrical damping may increase because of a larger cross sectional area of the mass and/or a change in electrical conductivity.
For a given geophone design, damping is inversely proportional to natural frequency in this sense:
If the damping of the geophone is near its optimum at a natural or resonant frequency of 14 Hz, then it will be overdamped at 7 Hz and underdamped at 28 Hz. Accordingly, some manufacturers offer geophones with either a brass or an aluminum bobbin. A brass bobbin adds mass to the coil assembly and reduces the damping. Brass bobbins are used for low frequency geophones, generally up to 8 Hz. The use of aluminum bobbins reduces the mass of the mass-coil assembly and increases the damping. Aluminum bobbins are used for higher frequency geophones. But, as previously mentioned, increasing or decreasing the mass of the bobbin, be it made from brass or aluminum, inversely affects the eddy-current damping produced by the bobbin if the cross-sectional area or the conductivity changes. For example, aluminum has higher conductivity per unit of mass than either copper or brass and, therefore, the eddy-current damping is increased when changing from a brass to an aluminum bobbin having the same mass, and vice versa. Other manufacturers provide weight rings of different sizes that may be affixed to the bobbin, to adjust the damping.
In summary, a mass may be applied to the bobbin of a geophone to provide a mechanical damping force that is inversely proportional to the size of the mass. If, as is usual, the mass is a closed metallic weight ring or cylinder, an opposing electrical damping force is developed due to the induced EMF and to Foucault or eddy-currents that are generated when the ring cuts the magnetic lines of force. Thus, if a larger weight ring is applied to the moving element to reduce the mechanical damping force, the decreased resistance and larger cross-sectional area cause an increase in the induced current and an increase in eddy-current flow, both of which increase the damping force.
Use of weight rings is quite common in the prior-art. For example, U.S. Pat. No. 3,582,875 to Van Wambeck shows a mass 87 applied to bobbin 7 (FIG. 1 of the patent) to adjust the peak frequency (a function of the damping coefficient) of the geophone.
In U.S. Pat. No. 3,913,063 to Sears, a mandrel 14' is fitted inside bobbin 14. (FIGS. 1 and 2 of the patent) to act as a damping mass.
In very low frequency seisometers such as the HS-10, 2-Hz geophone, made by Geospace, Inc. of Houston, Tex., weight rings are supplied to grossly tune the geophone frequency to an approximate desired value. If the weight rings form closed circuits, both induced and eddy-currents will generate electrical-damping counter forces. But even if the weight rings are split (open circuit) eddy-current electrical damping is still present.
Thus, in the prior art, the mechanical damping force can be changed step-wise in gross steps. But with each gross stepwise change in the mechanical damping force, the geophone designer must struggle with the opposing electrical damping force. Earlier inventors of geophones have not disclosed means for making a vernier weight ring that is substantially immune to the concomitant opposing changes in the electrical damping force.